Chapter 12 & 13

1. Chapter 12 & 13 The Cell Cycle

2. Figure 12.1 Overview: The Key Roles of Cell Division • The continuity of life • Is based upon the reproduction of cells, or cell division

3. 100 µm An amoeba, a single-celled eukaryote, is dividing into two cells. Each new cell will be an individual organism. • Unicellular organisms often reproduce by mitotic cell division (asexual) • …but so do others Spores Vegetative propagation Animation of bacterial DNA replication = Binary fission. budding

4. 200 µm 20 µm (b) Growth and development. This micrograph shows a sand dollar embryo shortly after the fertilized egg divided, forming two cells (LM). (c) Tissue renewal. These dividing bone marrow cells (arrow) will give rise to new blood cells (LM). Figure 12.2 B, C • Multicellular organisms depend on cell division for • Development from a fertilized cell • Growth • Repair

5. CELL DIVISION • MITOTIC cell division results in genetically identical daughter cells • Cells duplicate their genetic material • Before they divide, ensuring that each daughter cell receives an exact copy of the genetic material, DNA • Is an integral part of the cell cycle

6. 50 µm Cellular Organization of the Genetic Material • A cell’s endowment of DNA, its genetic information • Is called its genome • The DNA molecules in a cell • Are packaged into chromosomes

7. Eukaryotic chromosomes • Consist of chromatin, a complex of DNA and protein that condenses during cell division • In animals • Somatic cells have two sets of chromosomes • Diploid (2n) • Gametes have one set of chromosomes • Haploid (n)

8. 0.5 µm A eukaryotic cell has multiplechromosomes, one of which is represented here. Before duplication, each chromosomehas a single DNA molecule. Chromosomeduplication(including DNA synthesis) Once duplicated, a chromosomeconsists of two sister chromatidsconnected at the centromere. Eachchromatid contains a copy of the DNA molecule. Centromere Sisterchromatids Separation of sister chromatids Mechanical processes separate the sister chromatids into two chromosomes and distribute them to two daughter cells. Centromeres Sister chromatids Figure 12.4 Distribution of Chromosomes During Cell Division Each duplicated chromosome has two sister chromatids, which separate during cell division • In preparation for cell division • DNA is replicated and the chromosomes condense

9. Eukaryotic cell division consists of • Mitosis, the division of the nucleus • Cytokinesis, the division of the cytoplasm • In meiosis • Sex cells are produced after a reduction in chromosome number

10. INTERPHASE S(DNA synthesis) G1 CytokinesisMitosis G2 MITOTIC(M) PHASE Figure 12.5 Phases of the Cell Cycle • The cell cycle consists of • The mitotic phase • Interphase Notice the time spent in interphase

11. What happens during Interphase? • Interphase can be divided into subphases • G1 phase • S phase • G2 phase The G0 phase (referred to the G zero phase) or resting phase is a period in the cell cycle in which cells exist in a quiescent state. G0 phase is viewed as either an extended G1 phase, where the cell is neither dividing nor preparing to divide, or a distinct quiescent stage that occurs outside of the cell cycle.Some types of cells, such as nerve and heart muscle cells, become quiescent when they reach maturity.

12. MITOSIS • The mitotic phase • Is made up of mitosis and cytokinesis

13. G2 OF INTERPHASE PROMETAPHASE PROPHASE Centrosomes(with centriole pairs) Aster Fragmentsof nuclearenvelope Early mitoticspindle Kinetochore Chromatin(duplicated) Centromere Nonkinetochoremicrotubules Kinetochore microtubule Chromosome, consistingof two sister chromatids Nuclearenvelope Plasmamembrane Nucleolus Figure 12.6 PMAT • Mitosis consists of four distinct phases • Prophase

14. METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS Metaphaseplate Cleavagefurrow Nucleolusforming Nuclear envelopeforming Daughter chromosomes Centrosome at one spindle pole Spindle Figure 12.6 • Metaphase • Anaphase • Telophase

15. The Mitotic Spindle: A Closer Look • The mitotic spindle • Is an apparatus of microtubules that controls chromosome movement during mitosis • The spindle arises from the centrosomes • And includes spindle microtubules and asters

16. Aster Centrosome MetaphasePlate Sisterchromatids Kinetochores Overlappingnonkinetochoremicrotubules Kinetochores microtubules 0.5 µm Microtubules Chromosomes Figure 12.7 Centrosome 1 µm • Some spindle microtubules • Attach to the kinetochores of chromosomes and move the chromosomes to the metaphase plate

17. EXPERIMENT 1 The microtubules of a cell in early anaphase were labeled with a fluorescent dye that glows in the microscope (yellow). Kinetochore Spindlepole Figure 12.8 • In anaphase, sister chromatids separate • And move along the kinetochore microtubules toward opposite ends of the cell

18. Nonkinetechore microtubules from opposite poles • Overlap and push against each other, elongating the cell • In telophase • Genetically identical daughter nuclei form at opposite ends of the cell

19. Cleavage furrow 100 µm Contractile ring of microfilaments Daughter cells Figure 12.9 A (a) Cleavage of an animal cell (SEM) Cytokinesis: A Closer Look • In animal cells • Cytokinesis occurs by a process known as cleavage, forming a cleavage furrow

20. Vesiclesforming cell plate Wall of patent cell 1 µm Cell plate New cell wall Daughter cells Figure 12.9 B (b) Cell plate formation in a plant cell (SEM) • In plant cells, during cytokinesis • A cell plate forms

21. 2 3 5 1 4 Chromatinecondensing Nucleus Chromosome Nucleolus Metaphase. The spindle is complete,and the chromosomes,attached to microtubulesat their kinetochores, are all at the metaphase plate. Prophase. The chromatinis condensing. The nucleolus is beginning to disappear.Although not yet visible in the micrograph, the mitotic spindle is staring to from. Prometaphase.We now see discretechromosomes; each consists of two identical sister chromatids. Laterin prometaphase, the nuclear envelop will fragment. Telophase. Daughternuclei are forming. Meanwhile, cytokinesishas started: The cellplate, which will divided the cytoplasm in two, is growing toward the perimeterof the parent cell. Anaphase. Thechromatids of each chromosome have separated, and the daughter chromosomesare moving to the ends of cell as their kinetochoremicrotubles shorten. Figure 12.10 • Mitosis in a plant cell

22. Mitosis and cytokinesis narrated animation • How the Cell Cycle Works narrated animation

23. G1 checkpoint Control system S G1 G2 M M checkpoint Figure 12.14 G2 checkpoint The Cell Cycle Control System • The sequential events of the cell cycle • Are directed by a distinct cell cycle control system, which is similar to a clock

24. G0 G1 checkpoint G1 G1 (a) If a cell receives a go-ahead signal at the G1 checkpoint, the cell continues      on in the cell cycle. (b) If a cell does not receive a go-ahead signal at the G1checkpoint, the cell exits the cell cycle and goes into G0, a nondividing state. Figure 12.15 A, B • The clock has specific checkpoints • Where the cell cycle stops until a go-ahead signal is received

25. Regulation of the Cell Cycle • The frequency of cell division • Varies with the type of cell • The cell cycle is regulated by a molecular control system • Molecules present in the cytoplasm • Regulate progress through the cell cycle • Two types of regulatory proteins are involved in cell cycle control • Cyclins • Cyclin-dependent kinases (Cdks) • Narrated animation “Control of the Cell Cycle”

26. Cell division is tightly controlled by complexes made of several specific proteins. • These complexes contain enzymes called cyclin-dependent kinases (CDKs), which turn on or off the various processes that take place in cell division. • CDK partners with a family of proteins called cyclins. • One such complex is mitosis-promoting factor (MPF), sometimes called maturation-promoting factor, which contains cyclin A or B and cyclin-dependent kinase (CDK). (See Figure 2a.) • CDK is activated when it is bound to cyclin, interacting with various other proteins that, in this case, allow the cell to proceed from G2 into mitosis. • The levels of cyclin change during the cell cycle (Figure 2b). In most cases, cytokinesis follows mitosis. Narrated animation: Cell Proliferation Signaling Pathway http://highered.mcgraw-hill.com/olc/dl/120073/bio15.swf

27. As shown in Figure 3, different CDKs are produced during the phases. The cyclins determine which processes in cell division are turned on or off and in what order by CDK. As each cyclin is turned on or off, CDK causes the cell to move through the stages in the cell cycle.

28. Both internal and external signalsControl the cell cycle checkpoints • There are three checkpoints a cell must pass through: the G1 checkpoint, G2 checkpoint, and the M-spindle checkpoint (Figure 4). • At each of the checkpoints, the cell checks that it has completed all of the tasks needed and is ready to proceed to the next step in its cycle. • Cells pass the G1 checkpoint when they are stimulated by appropriate external growth factors; for example, platelet-derived growth factor (PDGF) stimulates cells near a wound to divide so that they can repair the injury. • The G2 checkpoint checks for damage after DNA is replicated, and if there is damage, it prevents the cell from going into mitosis. • The M-spindle (metaphase) checkpoint assures that the mitotic spindles or microtubules are properly attached to the kinetochores (anchor sites on the chromosomes). If the spindles are not anchored properly, the cell does not continue on through mitosis. • The cell cycle is regulated very precisely. • Mutations in cell cycle genes that interfere with proper cell cycle control are found very often in cancer cells.

29. 1 3 2 EXPERIMENT Scalpels A sample of connective tissue was cut up into small pieces. Petriplate Enzymes were used to digest the extracellular matrix,resulting in a suspension of free fibroblast cells. Cells were transferred to sterile culture vessels containing a basic growth medium consisting of glucose, amino acids, salts, and antibiotics (as a precaution against bacterial growth). PDGF was added to half the vessels. The culture vessels were incubated at 37°C. Without PDGF Figure 12.17 With PDGF Growth Factor proteins • Growth factors • Stimulate other cells to divide

30. (a) Normal mammalian cells. The availability of nutrients, growth factors, and a substratum for attachment limits cell density to a single layer. Cells anchor to dish surface and divide (anchorage dependence). When cells have formed a complete single layer, they stop dividing (density-dependent inhibition). If some cells are scraped away, the remaining cells divide to fill the gap and then stop (density-dependent inhibition). Figure 12.18 A 25 µm Environmental effects on cell cycling • In density-dependent inhibition • Crowded cells stop dividing • Most animal cells exhibit anchorage dependence • In which they must be attached to a substratum to divide

31. Cancer cells do not exhibitanchorage dependence or density-dependent inhibition. Cancer cells. Cancer cells usually continue to divide well beyond a single layer, forming a clump of overlapping cells. (b) 25 µm • Cancer cells • Exhibit neither density-dependent inhibition nor anchorage dependence Most oncogenesare mutations of certain normal genes calledproto-oncogenes. Proto-oncogenes are the "good" genes that normally control what kind of cell it is and how often it divides. When a proto-oncogene mutates (changes) into an oncogene, it becomes a "bad" gene that can become permanently turned on or activated when it is not supposed to be.When this happens, the cell grows out of control, which can lead to cancer.

32. Loss of cell cycle control • How cell division (and thus tissue growth) is controlled is very complex. The following terms are some of the features that are important in regulation, and places where errors can lead to cancer. • Cancer is a disease where regulation of the cell cycle goes awry and normal cell growth and behavior is lost. • Cdk (cyclin dependent kinase, adds phosphate to a protein), along with cyclins, are major control switches for the cell cycle, causing the cell to move from G1 to S or G2 to M. • MPF (Mitosis Promoting Factor) includes the CdK and cyclins that triggers progression through the cell cycle. • p53 is a protein that functions to block the cell cycle if the DNA is damaged. If the damage is severe this protein can cause apoptosis (cell death). • p53 levels are increased in damaged cells. This allows time to repair DNA by blocking the cell cycle. • A p53 mutation is the most frequent mutation leading to cancer. An extreme case of this is Li Fraumeni syndrome, where a genetic a defect in p53 leads to a high frequency of cancer in affected individuals. • p27 is a protein that binds to cyclin and Cdks, blocking entry into S phase. Recent research (Nature Medicine 3, 152 (1997)) suggests that breast cancer prognosis is determined by p27 levels. Reduced levels of p27 predict a poor outcome for breast cancer patients.

33. Loss of Cell Cycle Controls in Cancer Cells • Cancer cells • Do not respond normally to the body’s control mechanisms • Form tumors P53 is a tumor suppressor protein that in humans is encoded by the TP53 gene Ras proteins also play a role in cell growth and division. Overactive Ras signaling can ultimately lead to cancer (rat sarcoma) BRCA1 and BRCA 2 are tumor-suppressor genes expressed in the cells of breast and other tissue, where it helps repair damaged DNA, or destroy cells if DNA cannot be repaired. If BRCA itself is damaged, damaged DNA is not repaired properly and this increases risks for cancers

34. 4 3 2 1 Lymphvessel Tumor Bloodvessel Glandular tissue Cancer cell MetastaticTumor A small percentage of cancer cells may survive and establish a new tumor in another part of the body. Cancer cells spread through lymph and blood vessels to other parts of the body. A tumor grows from a single cancer cell. Cancer cells invade neighboring tissue. • Malignant tumors invade surrounding tissues and can metastasize • Exporting cancer cells to other parts of the body where they may form secondary tumors Figure 12.19

35. The Case of Henrietta Lacks • HeLa cells and their legacy 1920–1951 • Who was she? Familiarize yourself with her biography. • What was her significance to medical science? • What were the ethical questions her case raises?

36. Meiosis Overview • Concept 13.3: Meiosis reduces the number of chromosome sets from diploid to haploid • This maintains the correct # of chromosomes in each organism of a species Meiosis introduces genetic variation into offspring (without changing the chromosome #)

37. Meiosis leads to greater genetic variation in offspring Concept 13.4: Genetic variation produced in sexual life cycles contributes to evolution • Reshuffling of genetic material during synapsis of Meiosis I (crossing over) animation • Independent assortment (animation) of homologues and chromatids ensures that gametes will all be unique • Produces genetic variation • How does this ensure natural selection can take place? Examples?

38. Overview • Meiosis takes place in two sets of divisions • Meiosis I • Replicates the diploid number of chromosomes and recombines them randomly • Meiosis II • Reduces the number of chromosomes from diploid to haploid • Narrated animation of the stages of meiosis

39. MEIOSIS I: Separates homologous chromosomes INTERPHASE PROPHASE I METAPHASE I ANAPHASE I Sister chromatids remain attached Centromere (with kinetochore) Centrosomes (with centriole pairs) Chiasmata Metaphase plate Sister chromatids Spindle Nuclear envelope Homologous chromosomes separate Microtubule attached to kinetochore Tetrad Chromatin Pairs of homologous chromosomes split up Chromosomes duplicate Tertads line up Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example • Interphase and meiosis I Figure 13.8

40. MEIOSIS II: Separates sister chromatids TELOPHASE II AND CYTOKINESIS TELOPHASE I AND CYTOKINESIS METAPHASE II ANAPHASE II PROPHASE II Cleavage furrow Haploid daughter cells forming Sister chromatids separate Two haploid cells form; chromosomes are still double During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes Figure 13.8 • Telophase I, cytokinesis, and meiosis II

41. Spermatogenesis and Oogenesis

42. MITOSIS MEIOSIS Chiasma (site of crossing over) Parent cell (before chromosome replication) MEIOSIS I Prophase I Prophase Chromosome replication Chromosome replication Tetrad formed by synapsis of homologous chromosomes Duplicated chromosome (two sister chromatids) 2n = 6 Tetrads positioned at the metaphase plate Chromosomes positioned at the metaphase plate Metaphase I Metaphase Sister chromatids separate during anaphase Anaphase Telophase Homologues separate during anaphase I; sister chromatids remain together Anaphase I Telophase I Haploid n = 3 Daughter cells of meiosis I 2n 2n MEIOSIS II Daughter cells of mitosis n n n n Daughter cells of meiosis II Sister chromatids separate during anaphase II Figure 13.9 SUMMARY: Mitosis vs Meiosis • Similarities? • Differences? • Animated comparison between mitosis and meiosis

43. A Comparison of Mitosis and Meiosis Meiosis and mitosis can be distinguished from mitosis by three events in Meiosis l • Synapsis and crossing over • Homologous chromosomes physically connect and exchange genetic information • Tetrads on the metaphase plate • At metaphase I of meiosis, paired homologous chromosomes (tetrads) are positioned on the metaphase plates • Separation of homologues • At anaphase I of meiosis, homologous pairs move toward opposite poles of the cell • In anaphase II of meiosis, the sister chromatids separate • Flash animation of these events